A placebo is a placebo is a placebo ... or maybe not, a new study suggests

We've all heard of the placebo effect -- thinking a simulated treatment has an effect -- but what exactly is in that placebo, anyway, and could it have a noticeable effect? A new study published Monday in the Annals of Internal Medicine analyzed numerous research trials to find out, but discovered that placebo disclosure is rare.

Many drug trials involve a placebo, a sham drug whose results are compared with the results of the real medication. A placebo is supposed to contain a harmless substance, such as sugar or vegetable oil, which has no significant effect on the body. In this study, researchers delved into 176 studies published in reputable medical journals, such as the New England Journal of Medicine, the Journal of the American Medical Assn. and the Annals of Internal Medicine, from January 2008 to December 2009 to see if placebo contents were disclosed and if so, what they were.

In the mix were 86 studies of pills, 65 of injections and 25 of other treatments. The vast majority of studies didn't mention the contents of the placebos, with pill studies providing the least information. Only 8.2% of pill studies disclosed ingredients, compared with 26.7% of studies using injections and other treatments.

The study authors argue that placebo ingredients may not always be as inconsequential as some may think. They write: "For instance, olive oil and corn oil have been used as the placebo in trials of cholesterol-lowering drugs. This may lead to an understatement of drug benefit: The monounsaturated and polyunsaturated fatty acids of these 'placebos,' and their antioxidant and anti-inflammatory effects, can reduce lipid levels and heart disease."

Certain placebos, they add, may skew results in favor of the active drug. The researchers referenced a trial for a drug used to treat anorexia linked with cancer in which a lactose placebo was used. Since lactose intolerance is common among cancer patients, the fact that some suffered stomach problems from the placebo may have made the actual drug look more beneficial.

"Perfect placebo is not the aim," they write, "rather, we seek to ensure that its composition is disclosed."

The authors suggest that prominent journals begin to require studies to include placebo disclosure, and propose that the Consolidated Standards of Reporting Trials (which offers recommendations for improving reporting of randomized control trials) change its guidelines to advocate disclosure of placebo ingredients.